#1066 · Algebra · Sequences & series

Triangular pyramidal cannonballs

A triangular pyramid of cannonballs has 18 balls along each edge of the triangular base. Find the total number of balls in the pyramid.

~2 minAnswer checked instantlytetrahedral numberssum of triangular numbers

H. S. Hall & S. R. Knight, Higher Algebra (1891) · Ch. VI. Harmonical Progression, Examples VI.b, No. 2

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Checked against the answer in H. S. Hall & S. R. Knight, Higher Algebra (1891).